This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
A small, mostly Sephardic Jewish population, numbering roughly fifteen thousand by 1840, had grown into a settled American community over the first decades of the nineteenth century, but the decisive transformation came later, in a wave of Yiddish speaking Ashkenazi immigration out of Eastern Europe. About two million people arrived in the United States between 1880 and the outbreak of the First World War in 1914, fleeing poverty and, particularly from the Russian Empire, organised violence against Jewish communities.
The wave continued past the war's outbreak and on into the 1920s, until the Immigration Act of 1924 imposed national origin quotas that sharply curtailed further arrivals. By the time that law took effect, the United States had become home to one of the largest Jewish populations anywhere in the world, built almost entirely within a single generation of mass immigration.
What this changed for Judaism's spread was the rapid creation of a major new center of Jewish life in North America, orders of magnitude larger than the small Sephardic community that had preceded it. The two million arrivals of 1880 to 1914 transformed the scale of Judaism's American presence within a single generation, before the 1924 quotas closed the wave that had built it.